Your quotation has distorted the original a bit. (Well, of course the original is in French, but Lewis Galantière’s translation into English is also lovely.) It’s from chapter 3 of the book Wind, Sand & Stars (in French, Terre des hommes). Here’s the full text of that chapter, which I highly recommend to everyone: https://gist.github.com/anonymous/cd950f8ac6d58349e9e6

And here’s the paragraph surrounding the quotation in question (if I remember correctly the paragraphs are arranged a bit differently in the French original, but anyway...):

It is as if there were a natural law which ordained that to achieve this end, to refine the curve of a piece of furniture, or a ship’s keel, or the fuselage of an airplane, until gradually it partakes of the elementary purity of the curve of a human breast or shoulder, there must be the experimentation of several generations of craftsmen. In anything at all, perfection is finally attained not when there is no longer anything to add, but when there is no longer anything to take away, when a body has been stripped down to its nakedness.

Thank you, I was just searching for the provenance of that quote.